Intrabody navigation and imaging system for medical applications

ABSTRACT

A system and method for tracking the position and orientation of a probe such as a catheter whose transverse inner dimension may be at most about two millimeters. Three planar antennas that at least partly overlap are used to transmit electromagnetic radiation simultaneously, with the radiation transmitted by each antenna having its own spectrum. In the case of single-frequency spectra, the antennas are provided with mechanisms for decoupling them from each other. A receiver inside the probe includes sensors of the three components of the transmitted field, with sensors for at least two of the three components being pairs of sensors, such as coils, disposed symmetrically with respect to a common reference point. In one variant of the receiver, the coils are collinear and are wound about cores that are mounted in pairs of diametrically opposed apertures in the housing of the probe. In another variant of the receiver-catheter combination, the catheter is configured with an inner and outer sleeve connected at their ends by one or more flexible elements on which the coils are mounted. Each member of a pair of coils that sense the same component of the transmitted field is connected to a different input of a differential amplifier. The position and orientation of the receiver relative to the antennas are determined noniteratively, by setting up an overdetermined set of linear equations that relates the received signals to transmitter-receiver amplitudes, solving for the amplitudes and inferring the position coordinates and the orientation angles of the receiver relative to the transmitter from these amplitudes. For the purpose of navigating the probe within the body of a patient, the antennas are rigidly attached to an imaging device such as a fluoroscope, to provide a common frame of reference for both the acquired images of the patient and the measured position and orientation of the receiver. Alternatively, a second receiver is rigidly attached to the imaging device to provide the common frame of reference. In a third alternative, a third receiver is rigidly attached to the patient during imaging, and the mutual positions and orientations of the patient and the imaging device thus measured are used in subsequent navigation of the probe within the body of the patient. An imaging device that includes an electrically conductive surface is provided with a magnetically permeable compensator for minimizing distortion of the transmitted field. A scheme is provided for retrofitting an apparatus such as the receiver to a prior art catheter.

[0001] This is a Divisional of U.S. Ser. No. 09/463,177, pending.

FIELD AND BACKGROUND OF THE INVENTION

[0002] The present invention relates to electromagnetic tracking devicesand, more particularly, to a system and method for tracking a medicalprobe such as a catheter as the probe is moved through the body of apatient.

[0003] It is known to track the position and orientation of a movingobject with respect to a fixed frame of reference, by equipping themoving object with a transmitter that transmits electromagneticradiation, placing a receiver in a known and fixed position in the fixedframe of reference, and inferring the continuously changing position andorientation of the object from signals transmitted by the transmitterand received by the receiver. Equivalently, by the principle ofreciprocity, the moving object is equipped with a receiver, and atransmitter is placed in a known and fixed position in the fixed frameof reference. Typically, the transmitter includes three orthogonalmagnetic dipole transmitting antennas; the receiver includes threeorthogonal magnetic dipole receiving sensors; and the object is closeenough to the stationary apparatus (transmitter or receiver), and thefrequencies of the signals are sufficiently low, that the signals arenear field signals. Also typically, the system used is a closed loopsystem: the receiver is hardwired to, and explicitly synchronized with,the transmitter. Representative prior art patents in this field includeU.S. Pat. No. 4,287,809 and U.S. Pat. No. 4,394,831, to Egli et al.;U.S. Pat. No. 4,737,794, to Jones; U.S. Pat. No. 4,742,356, to Kuipers;U.S. Pat. No. 4,849,692, to Blood; and U.S. Pat. No. 5,347,289, toElhardt. Several of the prior art patents, notably Jones, presentnon-iterative algorithms for computing the position and orientation ofmagnetic dipole transmitters with respect to magnetic dipole receivers.

[0004] An important variant of such systems is described in U.S. Pat.No. 5,600,330, to Blood. In Blood's system, the transmitter is fixed inthe fixed reference frame, and the receiver is attached to the movingobject. Blood's transmitting antennas are spatially extended, and socannot be treated as point sources. Blood also presents an algorithmwhich allows the orientation, but not the position, of the receiverrelative to the transmitter to be calculated non-iteratively.

[0005] Systems similar to Blood's are useful for tracking a probe, suchas a catheter or an endoscope, as that probe is moved through the bodyof a medical patient. It is particularly important in this applicationthat the receiver be inside the probe and that the transmitter beexternal to the patient, because transmitting antennas of sufficientpower would not fit inside the confined volume of the probe. Arepresentative prior art system of this type is described in PCTPublication WO 96/05768, entitled “Medical Diagnosis, Treatment andImaging Systems”, which is incorporated by reference for all purposes asif fully set forth herein. Medical applications of such systems includecismyocardial revascularization, balloon catheterization, stentemplacement, electrical mapping of the heart and the insertion of nervestimulation electrodes into the brain.

[0006] Perhaps the most important application of this tracking is tointrabody navigation, as described by Acker in U.S. Pat. No. 5,729,129,with reference to PCT Publication No. WO 95/09562. A three-dimensionalimage, such as a CT or MRI image, of the patient is acquired. This imageincludes fiducial markers at predetermined fiducial points on thesurface of the patient. Auxiliary receivers similar to the receiver ofthe probe are placed at the fiducial points. The signals received by theauxiliary receivers are used to register the image with respect to thetransmitter frame of reference, so that an icon that represents theprobe can be displayed, superposed on a slice of the image, with thecorrect position and orientation with respect to the image. In this way,a physician can see the position and orientation of the probe withrespect to the patient's organs.

[0007] WO 96/05768 illustrates another constraint imposed on suchsystems by the small interior dimensions of the probe. In most prior artsystems, for example, the system of Egli et al., the receiver sensorsare three concentric orthogonal coils wound on a ferrite core. The coilsare “concentric” in the sense that their centers coincide. Such areceiver of sufficient sensitivity would not fit inside a medical probe.Therefore, the sensor coils of WO 96/05768 are collinear: the threeorthogonal coils are positioned one behind the other, with their centerson the axis of the probe, as illustrated in FIG. 3 of WO 96/05768. Thisreduces the accuracy of the position and orientation measurements,because instead of sensing three independent magnetic field componentsat the same point in space, this receiver senses three independentmagnetic field components at three different, albeit closely spaced,points in space.

[0008] A further, consequent concession of the system of WO 96/05768 tothe small interior dimensions of a catheter is the use of coils wound onair cores, rather than the conventional ferrite cores. The high mutualcoupling of collinear coils wound on ferrite cores and measuring threeindependent field components at three different points in space woulddistort those measurements sufficiently to make those measurementsfatally nonrepresentative of measurements at a single point.

[0009] Another drawback of the system of WO 96/05768 relates to thegeometry of the transmitter antennas. These are three nonoverlappingflat coplanar coils, preferably arranged in a triangle. Because thestrength of the field transmitted by one of these coils falls as thereciprocal cube of the distance from the coil, the receiver usuallysenses fields of very disparate strength, which further degrades theaccuracy of the position and orientation measurements. Acker addressesthis problem by automatically boosting the power supplied totransmitting coils far from the receiver. In U.S. Pat. No. 5,752,513,Acker et al. address this problem by overlapping the coplanartransmitting coils.

[0010] Acker et al. transmit time-multiplexed DC signals. This timemultiplexing slows down the measurement. Frequency multiplexing, astaught in WO 96/05768, overcomes this problem, but introduces a newproblem insofar as the transmitting coils are coupled by mutualinductance at non-zero transmission frequency, so that the transmittedfield geometry is not the simple geometry associated with a single coil,but the more complex geometry associated with several coupled coils.This complicates and slows down the calculation of the position andorientation of the receiver relative to the transmitter coils. PCTPublication WO 97/36143, entitled “Mutual Induction Correction”,addresses this problem by generating, at each transmitter coil,counter-fields that cancel the fields generated by the other transmittercoils.

[0011] A further source of slowness in calculating the position andorientation of the receiver is the iterative nature of the calculationrequired for a spatially extended transmitter. As noted above, Bloodcalculates the position of the receiver iteratively. Even in the DCcase, Acker et al. calculate both the position and the orientation ofthe receiver iteratively.

[0012] There is thus a widely recognized need for, and it would behighly advantageous to have, a faster and more accurate method fortracking a medical probe inside the body of a patient.

SUMMARY OF THE INVENTION

[0013] According to the present invention there is provided a system fortracking a position and an orientation of a probe, including a pluralityof first sensors, each of the first sensors for detecting a differentcomponent of a vector force field, each of the first sensors includingtwo sensor elements disposed symmetrically about a common referencepoint in the probe, the first sensors being mounted inside the probe.

[0014] According to the present invention there is provided a method fordetermining a position and an orientation of an object with respect to areference frame, including the steps of: (a) providing the object withthree independent sensors of electromagnetic radiation; (b) providingthree independent transmitting antennas of the electromagneticradiation, each of the transmitting antennas having a fixed position inthe reference frame, at least one of the transmitting antennas beingspatially extended; (c) transmitting the electromagnetic radiation,using the transmitting antennas, a first of the transmitting antennastransmitting the electromagnetic radiation of a first spectrum, a secondof the transmitting antennas transmitting the electromagnetic radiationof a second spectrum independent of the first spectrum, and a third ofthe transmitting antennas transmitting the electromagnetic radiation ofa third spectrum independent of the first spectrum; (d) receivingsignals corresponding to the electromagnetic radiation, at all three ofthe sensors, at a plurality of times, in synchrony with the transmittingof the electromagnetic radiation; and (e) inferring the position and theorientation of the object noniteratively from the signals.

[0015] According to the present invention there is provided a system fordetermining a position and an orientation of an object, including: (a) aplurality of at least partly overlapping transmitter antennas; (b) amechanism for exciting the transmitter antennas to transmitelectromagnetic radiation simultaneously, the electromagnetic radiationtransmitted by each of the transmitter antennas having a differentspectrum; (c) at least one electromagnetic field sensor, associated withthe object, operative to produce signals corresponding to theelectromagnetic radiation; and (d) a mechanism for inferring theposition and the orientation of the object from the signals.

[0016] According to the present invention there is provided a system fordetermining a position and an orientation of an object, including: (a) aplurality of at least partly overlapping transmitter antennas; (b) amechanism for exciting each of the transmitter antennas to transmitelectromagnetic radiation of a certain single independent frequency andphase, the mechanism including, for each of the transmitter antennas, amechanism for decoupling the each transmitter antenna from theelectromagnetic radiation transmitted by every other transmitterantenna; (c) at least one electromagnetic field sensor, associated withthe object, operative to produce signals corresponding to theelectromagnetic radiation; and (d) a mechanism for inferring theposition and the orientation of the object from the signals.

[0017] According to the present invention there is provided a catheter,including: (a) a housing having a transverse inner dimension of at mostabout two millimeters; and (b) at least one coil, wound about a solidcore, mounted inside the housing.

[0018] According to the present invention there is provided a system fornavigating a probe inside a body, including: (a) a receiver ofelectromagnetic radiation, inside the probe; (b) a device for acquiringan image of the body; and (c) a transmitter, of the electromagneticradiation, including at least one antenna rigidly attached to the deviceso as to define a frame of reference that is fixed with respect to thedevice.

[0019] According to the present invention there is provided a system fornavigating a probe inside a body, including: (a) a first receiver ofelectromagnetic radiation, inside the probe; (b) a device for acquiringan image of the body; and (c) a second receiver, of the electromagneticradiation, rigidly attached to the device so as to define a frame ofreference that is fixed with respect to the device.

[0020] According to the present invention there is provided a method ofnavigating a probe inside a body, including the steps of: (a) providinga device for acquiring an image of the body; (b) simultaneously: (i)acquiring the image of the body, and (ii) determining a position andorientation of the probe with respect to the image; and (c) displayingthe image of the body with a representation of the probe superposedthereon according to the position and the orientation.

[0021] According to the present invention there is provided a device forsensing an electromagnetic field at a point, including at least foursensing elements, at least two of the sensing elements being disposedeccentrically with respect to the point.

[0022] According to the present invention there is provided a method fordetermining a position and an orientation of an object with respect to areference frame, including the steps of: (a) providing the object withthree independent sensors of electromagnetic radiation; (b) providingthree independent transmitting antennas of the electromagneticradiation, each of the transmitting antennas having a fixed position inthe reference frame, at least one of the transmitting antennas beingspatially extended; (c) transmitting the electromagnetic radiation,using the transmitting antennas, a first of the transmitting antennastransmitting the electromagnetic radiation of a first spectrum, a secondof the transmitting antennas transmitting the electromagnetic radiationof a second spectrum independent of the first spectrum, and a third ofthe transmitting antennas transmitting the electromagnetic radiation ofa third spectrum independent of the first spectrum; (d) receivingsignals corresponding to the electromagnetic radiation, at all three ofthe sensors, at a plurality of times, in synchrony with the transmittingof the electromagnetic radiation; (e) setting up an overdetermined setof linear equations relating the signals to a set of amplitudes, therebeing, for each of the sensors: for each transmitting antenna: one ofthe amplitudes; and (f) solving the set of linear equations for theamplitudes.

[0023] According to the present invention there is provided a method ofnavigating a probe inside a body, including the steps of: (a) providinga device for acquiring an image of the body; (b) simultaneously: (i)acquiring the image of the body, and (ii) determining a position and anorientation of the body with respect to the image; (c) determining aposition and an orientation of the probe with respect to the body; and(d) displaying the image of the body with a representation of the probesuperposed thereon according to both of the positions and both of theorientations.

[0024] According to the present invention there is provided a device forsensing an electromagnetic field at a point, including: (a) two sensingelements, each of the sensing elements including a first lead and asecond lead, the first leads being electrically connected to each otherand to ground; and (b) a differential amplifier, each of the secondleads being electrically connected to a different input of thedifferential amplifier.

[0025] According to the present invention there is provided a catheterincluding: (a) an outer sleeve having an end; (b) an inner sleeve havingan end and slidably mounted within the outer sleeve; (c) a firstflexible member connecting the end of the outer sleeve to the end of theinner sleeve; and (d) a first coil mounted on the first flexible member.

[0026] According to the present invention there is provided a system fordetermining a position and an orientation of an object, including:(a) atleast one transmitter antenna for transmitting an electromagnetic field;(b) a first electromagnetic field sensor, associated with the object andincluding two sensing elements responsive to a first component of thetransmitted electromagnetic field, each of the sensing elementsincluding a first lead and a second lead, the first leads beingelectrically connected to each other and to ground; and (c) a firstdifferential amplifier, each of the second leads being electricallyconnected to a different input of the first differential amplifier.

[0027] According to the present invention there is provided an imagingdevice, including: (a) an electrically conducting surface; (b) amagnetically permeable compensator; and (c) a mechanism for securing thecompensator relative to the surface so as to substantially suppress adistortion of an external electromagnetic field caused by the surface.

[0028] According to the present invention there is provided a device forsensing an electromagnetic field, including: (a) a housing, including afirst pair of diametrically opposed apertures, (b) a first core mountedin the first pair of apertures; and (c) a first coil of electricallyconductive wire wound about the core.

[0029] According to the present invention there is provided a probe forinteracting with a body cavity, including: (a) a substantiallycylindrical catheter; (b) a satellite; and (c) a mechanism forreversibly securing the satellite at a fixed position and orientationrelative to the catheter after the catheter and the satellite have beeninserted into the body cavity.

[0030] Each receiver sensor of the present invention includes two sensorelements placed symmetrically with respect to a reference point insidethe probe. All the sensor element pairs share the same reference point,so that the measured magnetic field components are representative of thefield component values at the single reference point, instead of atthree different points, as in the prior art system, despite the confinedtransverse interior dimensions of the probe. Because of the symmetricdisposition of the sensor elements with respect to the reference point,the measured magnetic field components are representative of the fieldcomponents at the reference point, despite the individual sensingelements not being centered on the reference point. This property of notbeing centered on the reference point is termed herein an eccentricdisposition with respect to the reference point.

[0031] In one preferred embodiment of the receiver of the presentinvention, the sensor elements are helical coils. Within each sensor,the coils are mutually parallel and connected in series. As in the caseof the prior art receivers, the coils are arranged with their centers onthe axis of the probe. To ensure that coils of different sensors aremutually perpendicular, the probe housing includes mutuallyperpendicular pairs of diametrically opposed apertures formed therein,the coils whose axes are perpendicular to the axis of the probe arewound about cores whose ends extend past the ends of the respectivecoils, and the ends of the cores are mounted in their respectiveapertures.

[0032] In another preferred embodiment of the receiver of the presentinvention, with three sensors, the sensor elements are flat rectangularcoils bent to conform to the shape of the cylindrical interior surfaceof the probe. The sensor elements of the three sensors are interleavedaround the cylindrical surface. The advantage of this preferredembodiment over the first preferred embodiment is that this preferredembodiment leaves room within the probe for the insertion of othermedical apparati.

[0033] As noted above, within any one sensor, the coils are connected inseries. This connection is grounded. The other end of each coil isconnected, by one wire of a twisted pair of wires, to a different inputof a differential amplifier.

[0034] In a preferred embodiment of a cardiac catheter that incorporatesa receiver of the present invention, the catheter includes an innersleeve mounted slidably within an outer sleeve. One of the sensorsincludes two coils mounted within the inner sleeve, towards the distalend of the catheter. The distal end of the inner sleeve is connected tothe distal end of the outer sleeve by flexible strips. Each of the othersensors includes two coils mounted on opposed lateral edges of a pair offlexible strips that flank the inner sleeve, with the inner sleeverunning between the two members of the pair. When the inner sleeve is inthe extended position thereof relative to the outer sleeve, the flexiblestrips lie flat against the inner sleeve, and the catheter can bemaneuvered towards a patient's heart via the patient's blood vessels.When the end of the catheter has been introduced to the targeted chamberof the heart, the inner sleeve is withdrawn to the retracted positionthereof relative to the outer sleeve, and the pairs of flexible stripsform circles that are concentric with the reference point. Also mountedon the outward-facing surfaces of the flexible strips and, optionally,on the distal end of the inner sleeve, are electrodes forelectrophysiologic mapping of the heart. Alternatively, the electrode onthe distal end of the inner sleeve may be used for ablation of cardiactissue, for example in the treatment of ventricular tachycardia.

[0035] An alternative preferred embodiment of the cardiac catheter ofthe present invention has an inflatable balloon connecting the distalends of the inner and outer sleeves. The coils of the external sensorsare mounted on the external surface of the balloon. When the innersleeve is in the extended position thereof relative to the outer sleeve,the balloon lies flat against the inner sleeve, and the catheter can bemaneuvered towards the patient's heart via the patient's blood vessels.When the end of the catheter has been introduced to the targeted chamberof the heart, the inner sleeve is withdrawn to the retracted positionthereof relative to the outer sleeve, and the balloon is inflated to asphere that is concentric with the reference point.

[0036] Although the primary application of the receiver of the presentinvention is to tracking a probe by receiving externally generatedelectromagnetic radiation, the scope of the present invention includesreceivers for similar tracking based on the reception of any externallygenerated vector force field, for example, a time varying isotropicelastic field.

[0037] The algorithm of the present invention for inferring the positionand orientation of the receiver with respect to the transmitter issimilar to the algorithm described in co-pending Israel PatentApplication 122578. The signals received by the receiver are transformedto a 3×3 matrix M. The columns of M correspond to linear combinations ofthe amplitudes of the transmitted fields. The rows of M correspond tothe receiver sensors. A rotationally invariant 3×3 position matrix W anda 3×3 rotation matrix T are inferred noniteratively from the matrix M.The Euler angles that represent the orientation of the receiver relativeto the transmitter antennas are calculated noniteratively from theelements of T, and the Cartesian coordinates of the receiver relative tothe transmitter antennas are calculated from the elements of W. Apreliminary calibration of the system, either by explicitly measuringthe signals received by the receiver sensors at a succession ofpositions and orientations of the receiver, or by theoreticallypredicting these signals at the successive positions and orientations ofthe receiver, is used to determine polynomial coefficients that are usedin the noniterative calculation of the Euler angles and the Cartesiancoordinates. In essence, the extra time associated with an iterativecalculation is exchanged for the extra time associated with an initialcalibration. One simplification of the algorithm of the presentinvention, as compared to the algorithm of IL 122578, derives from thefact that the system of the present invention is a closed loop system.

[0038] The preferred arrangement of the transmitter antennas of thepresent invention is as a set of flat, substantially coplanar coils thatat least partially overlap. Unlike the preferred arrangement of Acker etal., it is not necessary that every coil overlap every other coil, aslong as each coil overlaps at least one other coil. The most preferredarrangement of the transmitter antennas of the present inventionconsists of three antennas. Two of the antennas are adjacent and definea perimeter. The third antenna partly follows the perimeter and partlyoverlaps the first two antennas. The elements of the first column of Mare sums of field amplitudes imputed to the first two antennas. Theelements of the second column of M are differences of field amplitudesimputed to the first two antennas. The elements of the third column of Mare linear combinations of the field amplitudes imputed to all threeantennas that correspond to differences between the field amplitudesimputed to the third antenna and the field amplitudes that would beimputed to a fourth antenna that overlaps the portion of the first twoantennas not overlapped by the third antenna.

[0039] The signals transmitted by the various antennas of the presentinvention have different, independent spectra. The term “spectrum”, asused herein, encompasses both the amplitude and the phase of thetransmitted signal, as a function of frequency. So, for example, if oneantenna transmits a signal proportional to cosωt and another antennatransmits a signal proportional to sinωt, the two signals are said tohave independent frequency spectra because their phases differ, eventhough their amplitude spectra both are proportional to δ(ω). The term“independent spectra”, as used herein, means that one spectrum is notproportional to another spectrum. So, for example, if one antennatransmits a signal equal to cosωt and another antenna transmits a signalequal to 2 cosωt, the spectra of the two signals are not independent.Although the scope of the present invention includes independenttransmitted signals that differ only in phase, and not in frequency, theexamples given below are restricted to independent transmitted signalsthat differ in their frequency content.

[0040] The method employed by the present invention to decouple thetransmitting antennas, thereby allowing each antennas to transmit atonly a single frequency different from the frequencies at which theother antennas transmit, or, alternatively, allowing two antennas totransmit at a single frequency but with a predetermined phaserelationship between the two signals, is to drive the antennas withcircuitry that makes each antenna appear to the fields transmitted bythe other antennas as an open circuit. To accomplish this, the drivingcircuitry of the present invention includes active circuit elements suchas differential amplifiers, unlike the driving circuitry of the priorart, which includes only passive elements such as capacitors andresistors. By “driving circuitry” is meant the circuitry that imposes acurrent of a desired transmission spectrum on an antenna, and not, forexample, circuitry such as that described in WO 97/36143 whose functionis to detect transmissions by other antennas with other spectra andgenerate compensatory currents.

[0041] With respect to intrabody navigation, the scope of the presentinvention includes the simultaneous acquisition and display of an imageof the patient and superposition on that display of a representation ofa probe inside the patient, with the representation positioned andoriented with respect to the image in the same way as the probe ispositioned and oriented with respect to the patient. This isaccomplished by positioning and orienting the imaging device withrespect to the frame of reference of the transmitter, in one of twoways. Either the transmitter antennas are attached rigidly to theimaging device, or a second receiver is attached rigidly to the imagingdevice and the position and orientation of the imaging device withrespect to the transmitter are determined in the same way as theposition and orientation of the probe with respect to the transmitterare determined. This eliminates the need for fiducial points andfiducial markers. The scope of the present invention includes both 2Dand 3D images, and includes imaging modalities such as CT, MRI,ultrasound and fluoroscopy. Medical applications to which the presentinvention is particularly suited include transesophagealechocardiography, intravascular ultrasound and intracardial ultrasound.In the context of intrabody navigation, the term “image” as used hereinrefers to an image of the interior of the patient's body, and not to animage of the patient's exterior.

[0042] Under certain circumstances, the present invention facilitatesintrabody navigation even if the image is acquired before the probe isnavigated through the patient's body with reference to the image. Athird receiver is attached rigidly to the limb of the patient to whichthe medical procedure is to be applied. During image acquisition, theposition and orientation of the third receiver with respect to theimaging device is determined as described above. This determines theposition and orientation of the limb with respect to the image.Subsequently, while the probe is being moved through the limb, theposition and orientation of the probe with respect to the limb isdetermined using the second method described above to position andorient the probe with respect to the imaging device during simultaneousimaging and navigation. Given the position and orientation of the probewith respect to the limb and the orientation and position of the limbwith respect to the image, it is trivial to infer the position andorientation of the probe with respect to the image.

[0043] Many imaging devices used in conjunction with the presentinvention include electrically conducting surfaces. One importantexample of such an imaging device is a fluoroscope, whose imageintensifier has an electrically conducting front face. According to thepresent invention, the imaging device is provided with a magneticallypermeable compensator to suppress distortion of the electromagneticfield near the electrically conducting surface as a consequence of eddycurrents induced in the electrically conducting surface by theelectromagnetic waves transmitted by the transmitting antennas of thepresent invention.

[0044] The scope of the present invention includes a scheme forretrofitting an apparatus such as the receiver of the present inventionto a catheter to produce an upgraded probe for investigating or treatinga body cavity of a patient. A tether provides a loose mechanicalconnection between the apparatus and the catheter while the apparatusand the catheter are inserted into the patient. When the apparatus andthe catheter reach targeted body cavity, the tether is withdrawn to pullthe apparatus into a pocket on the catheter. The pocket holds theapparatus in a fixed position and orientation relative to the catheter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

[0046]FIG. 1 is a schematic diagram of a system of the presentinvention;

[0047]FIG. 2A is a partly cut away perspective view of a probe and areceiver;

[0048]FIG. 2B is a circuit diagram of the receiver of FIG. 2A;

[0049]FIG. 2C illustrates features of the receiver of FIG. 2A thatsuppress unwanted electromagnetic coupling;

[0050]FIG. 3 is an axial sectional view of a probe and a receiver;

[0051]FIG. 4A shows two coils of opposite helicities;

[0052]FIG. 4 shows two coils of identical helicities;

[0053]FIG. 5 shows a second preferred embodiment of a receiver;

[0054]FIG. 6 is a plan view of three loop antennas and two phantom loopantennas;

[0055]FIGS. 7A, 7B and 7C show alternative configurations of pairedadjacent loop antennas;

[0056]FIG. 8 is a schematic block diagram of driving circuitry

[0057]FIG. 9 shows a C-mount fluoroscope modified for real-timeintrabody navigation

[0058]FIG. 10 shows a coil of the receiver of FIG. 5;

[0059]FIG. 11 shows a CT scanner modified for imaging in support ofsubsequent intracranial navigation;

[0060]FIG. 12A is a partly cut-away perspective view of a cardiaccatheter of the present invention in the retracted position thereof;

[0061]FIG. 12B is a perspective view of the catheter of FIG. 12A in theextended position thereof;

[0062]FIG. 12C is an end-on view of the catheter of FIG. 12a in theretracted position thereof;

[0063]FIG. 13A is a partly cut-away side view of a second embodiment ofthe cardiac catheter of the present invention in the retracted andinflated position thereof;

[0064]FIG. 13B is an end-on view of the catheter of FIG. 13A in theretracted and inflated position thereof;

[0065]FIG. 14 is a partial perspective view of the C-mount fluoroscopeof FIG. 9, including a magnetically permeable compensator;

[0066]FIG. 15 is a partial exploded perspective view of a preferredembodiment of the probe and receiver of FIG. 2A;

[0067]FIG. 16 illustrates a scheme for retrofitting an apparatus such asthe receiver of FIG. 2A to a catheter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] The present invention is of a system and method for tracking theposition and orientation of an object relative to a fixed frame ofreference. Specifically, the present invention can be used to track themotion of a medical probe such as a catheter or an endoscope within thebody of a patient.

[0069] The principles and operation of remote tracking according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

[0070] Referring now to the drawings, FIG. 1 illustrates, in generalterms, a system of the present invention. Within a probe 10 is rigidlymounted a receiver 14. Receiver 14 includes three field componentsensors 16, 18, and 20, each for sensing a different component of anelectromagnetic field. Sensor 16 includes two sensor elements 16 a and16 b. Sensor 18 includes two sensor elements 18 a and 18 b. Sensor 20includes two sensor elements 20 a and 20 b. Typically, the sensorelements are coils, and the sensed components are independent magneticfield components. Sensor elements 16 a and 16 b are on opposite sidesof, and equidistant from, a common reference point 22. Similarly, sensorelements 18 a and 18 b are on opposite sides of, and equidistant from,point 22, and sensor elements 20 a and 20 b also are on opposite sidesof, and equidistant from, point 22. In the illustrated example, sensors16, 18 and 20 are disposed collinearly along a longitudinal axis 12 ofprobe 10, but other configurations are possible, as discussed below.

[0071] The system of FIG. 1 also includes a transmitter 24 ofelectromagnetic radiation. Transmitter 24 includes three substantiallycoplanar rectangular loop antennas 26, 28 and 30 connected to drivingcircuitry 32. Loop antennas 26 and 28 are adjacent and are partlyoverlapped by loop antenna 30. Driving circuitry 32 includes appropriatesignal generators and amplifiers for driving each of loop antennas 26,28 and 30 at a different frequency. The electromagnetic waves generatedby transmitter 24 are received by receiver 14. The signals from receiver14 that correspond to these electromagnetic waves are sent to receptioncircuitry 34 that includes appropriate amplifiers and A/D converters.Reception circuitry 34 and driving circuitry 32 are controlled by acontroller/processor 36 that typically is an appropriately programmedpersonal computer. Controller/processor 36 directs the generation oftransmitted signals by driving circuitry 32 and the reception ofreceived signals by reception circuitry 34. Controller/processor 36 alsoimplements the algorithm described below to infer the position andorientation of probe 10. Note that the system of FIG. 1 is a closed-loopsystem: the reception of signals from receiver 14 is synchronized withthe transmission of electromagnetic waves by transmitter 24.

[0072]FIG. 2 shows a particular, slightly modified embodiment ofreceiver 14. FIG. 2A is a perspective, partly cut away view of probe 10with receiver 14 mounted in the housing 11 thereof. FIG. 2B is a circuitdiagram of receiver 14. In this embodiment, sensor elements 16 a, 16 b,18 a and 18 b are coils of conducting wire wound on ferrite cores 70.Coils 16 a and 16 b are mutually parallel. Coils 18 a and 18 b aremutually parallel and are perpendicular to coils 16 a and 16 b. Coils 16a, 16 b, 18 a and 18 b all are perpendicular to axis 12. Instead ofsensor 20 with two sensor elements 20 a and 20 b, the embodiment of FIG.2 has a single coil 20′ of conducting wire wound on a ferrite core 70.Coil 20′ is parallel to axis 12 and therefore is perpendicular to coils16 a, 16 b, 18 a and 18 b. Coil 20′ is centered on reference point 22.Sensors 16, 18 and 20′ are connected to reception circuitry 34 bytwisted wire pairs 38. As shown in the circuit diagram of FIG. 2B, coils16 a and 16 b are connected in series, and coils 18 a and 18 b areconnected in series.

[0073] Because sensors 16, 18 and 20′ of FIG. 2 all measure fieldcomponents at the same reference point 22, coils 16 a, 16 b, 18 a, 18 band 20′ can be wound on ferrite cores 70 instead of the air cores of WO96/05768 without causing undue distortion of the received signals,despite the small transverse interior diameter 72, typically less thantwo millimeters, of probe 10 when probe 10 is a catheter.

[0074] Wire pairs 38 are twisted in order to suppress electromagneticcoupling between wire pairs 38 and the environment, and in particular tosuppress electromagnetic coupling between wire pairs 38 and transmitter24. FIG. 2C is a circuit diagram that shows further features of thepresent invention that suppress this electromagnetic coupling. FIG. 2Cis drawn with particular reference to sensor 16, but the same featuresapply, mutatis mutandis, to sensor 18.

[0075] Coils 16 a and 16 b are connected in series by inner leads 116 aand 116 b thereof. Outer lead 216 a of coil 16 a is connected, by wire38 a of twisted wire pair 38, to a positive input 126 a of adifferential amplifier 128 of reception circuitry 34. Outer lead 216 bof coil 16 b is connected, by wire 38 b of twisted wire pair 38, to anegative input 126 b of differential amplifier 128. Inner leads 116 aand 116 b also are connected to ground 124 by a wire 122. Forillustrational clarity, wire 38 a is drawn as a solid line, wire 38 b isdrawn as a dotted line and wire 122 is drawn as a dashed line.

[0076]FIG. 15 is a partial exploded perspective view of a preferredembodiment of probe 10 and receiver 14. Housing 11 is substantiallycylindrical, with two recesses 511 and 513 incised therein. The boundaryof each recess 511 or 513 includes a pair of diametrically opposedapertures: apertures 510 and 512 in the boundary of recess 511 andapertures 514 and 516 in the boundary of recess 513. Arrows 530 and 532show two of the three components of a cylindrical coordinate system fordescribing position within and along housing 11. Arrow 530 points in thelongitudinal direction. Arrow 532 points in the azimuthal direction.Aperture pair 510, 512 is displaced both longitudinally and azimuthallyfrom aperture pair 514, 516.

[0077] Coil 16 a is a coil of electrically conducting wire that is woundabout a core 70 a. Core 70 a is mounted in apertures 514 and 516: end518 of core 70 a, that extends beyond coil 16 a, is mounted in aperture514 and is secured rigidly in place by a suitable glue, and end 520 ofcore 70 a, that extends beyond coil 16 a in the opposite direction, ismounted in aperture 516 and is secured rigidly in place by a suitableglue. Similarly, coil 18 a is a coil of electrically conducting wirethat is wound about a core 70 b. Core 70 b is mounted in apertures 510and 512: end 522 of core 70 b, that extends beyond coil 18 a, is mountedin aperture 510 and is secured rigidly in place by a suitable glue, andend 524 of core 70 b, that extends beyond coil 18 a in the oppositedirection, is mounted in aperture 512 and is secured rigidly in place bya suitable glue.

[0078]FIG. 15 also shows the preferred azimuthal separation of aperturepair 514, 516 from aperture pair 510, 512. Aperture pair 514, 516 isperpendicular to aperture pair 510, 512, in the sense that aperture pair514, 516 is displaced 90°, in the direction of arrow 532, from aperturepair 510, 512. This makes core 70 a perpendicular to core 70 b, andhence makes coils 16 a and 18 a mutually perpendicular.

[0079] In the case of probe 10 being a catheter for invasively probingor treating a body cavity such as a chamber of the heart, it ispreferable that housing 11 be made of a nonmagnetic metal such asnitinol, titanium, iconel, phynox or stainless steel. Housing 11 thus issufficiently flexible to bend under the lateral forces of the walls ofblood vessels through which probe 10 is inserted towards the bodycavity, and sufficiently resilient to return to its unstressed shape,with coils 16 a and 18 a mutually perpendicular, when the portion ofprobe 10 that includes receiver 14 reaches the interior of the bodycavity. Surprisingly, it has been found that the use of a conductivemetal as the material of housing 11 does not distort the electromagneticfield sensed by receiver 14 despite the current eddies induced inhousing 11 by the electromagnetic waves generated by transmitter 24.Apertures 510, 512, 514 and 516 are most conveniently formed by lasercutting. The accuracy of the mutual perpendicularity of coils 16 a and18 a obtained in this manner has been found to be superior to theaccuracy obtained by forming housing 11 as a solid cylindrical block anddrilling mutually perpendicular recesses in the block to receive coils16 a and 18 a.

[0080] Coils 16 b and 18 b are mounted similarly in similar pairs ofdiametrically opposed, azimuthally and longitudinally displacedapertures. This ensures that coils 16 a and 16 b are mutually parallel,that coils 18 a and 18 b are mutually parallel, and that coils 16 b and18 b are mutually perpendicular.

[0081] In an alternative structure (not shown) of housing 11, housing 11is formed as an open, spring-like frame that includes apertures 510,512, 514 and 516 in the form of small rings that are sized to accept theends 518, 520, 522 and 524 of cores 70 a and 70 b. The spring-likenature of this embodiment of housing 11 allows coils 16 a and 18 a to bemounted therein simply by forcing ends 518, 520, 522 and 524 into theirrespective apertures, and also allows housing 11 to flex duringinsertion towards a body cavity of a patient and to return to itsunstressed shape upon arrival inside the body cavity.

[0082]FIG. 3 is an axial sectional view of receiver 14 mounted in avariant of probe 10 that has two sections 10 a and 10 b connected by aflexible connector 40. As in FIG. 2, sensors 16 and 18 include sensorelements 16 a, 16 b, 18 a and 18 b that are coils of conducting wirewound on air cores and that are perpendicular to axis 12. Sensorelements 16 a and 16 b are mutually parallel, sensor elements 18 a and18 b are mutually parallel, and sensor elements 16 a and 16 b areperpendicular to sensor elements 18 a and 18 b. Sensor 20 includes twosensor elements: coils 20 a and 20 b of conducting wire wound on aircores. Coils 20 a and 20 b are equidistant from reference point 22 andare parallel to axis 12. Like coils 16 a and 16 b and like coils 18 aand 18 b, coils 20 a and 20 b are connected in series. Flexibleconnector 40 allows this variant of probe 10 to bend as this variant ofprobe 10 is moved within a medical patient. Sensor element pairs 16, 18and 20 are disposed symmetrically with respect to reference point 22 inthe sense that when probe 10 of FIG. 3 is straight, as drawn, sensorelements 16 a and 16 b are on opposite sides of, and equidistant from,reference point 22; and likewise sensor elements 18 a and 18 b are onopposite sides of, and are equidistant from, reference point 22; andsensor elements 20 a and 20 b are on opposite sides of, and areequidistant from, reference point 22. Note that when probe 10 of FIG. 3is straight, sensor elements 16 a, 16 b, 18 a, 18 b, 20 a and 20 b allare collinear, along axis 12 that intersects point 22, and so aredisposed symmetrically with respect to point 22.

[0083] For coil pairs such as pairs 16 a and 16 b to produce signalsrepresentative of a magnetic field component at point 22 when the coilpairs are connected as shown in FIG. 2A, the two coils must haveopposite helicity, as illustrated in FIG. 4A, so that, in a spatiallyuniform time varying magnetic field, the signals induced in the two coilpairs 16 a and 16 b reinforce each other instead of canceling eachother. Coil pairs 16 a and 16 b that have identical helicities, asillustrated in FIG. 4B, may be used to measure a magnetic fieldcomponent gradient at point 22. Alternatively, coil pairs of identicalhelicities may be used to measure magnetic field components if the topof one coil is connected to the bottom of the other coil.

[0084]FIG. 5 illustrates a second class of preferred embodiments ofreceiver 14. In FIG. 5, a conceptual cylindrical surface is denoted bydashed lines 42 and dashed circles 44. The embodiment of receiver 14illustrated in FIG. 5 includes three sensors 16, 18 and 20, each withtwo sensor elements 16 c and 16 d, 18 c and 18 d, and 20 c and 20 d,respectively. Each sensor element is a flat rectangular coil, of manyturns of conducting wire, that is bent into an arcuate shape to conformto the shape of the cylindrical surface. Sensor elements 16 c, 18 c and20 c are interleaved around circle 44 a. Sensor elements 16 d, 18 d and20 d are interleaved around circle 44 b. Sensor elements 16 c and 16 dare disposed symmetrically with respect to reference point 22, meaningthat sensor elements 16 c and 16 d are on opposite sides of referencepoint 22, are equidistant from reference point 22, and are oriented sothat an appropriate 180° rotation about point 22 maps sensor 16 c intosensor 16 d. Similarly, sensor elements 18 c and 18 d are disposedsymmetrically with respect to reference point 22, and sensor elements 20c and 20 d are disposed symmetrically with respect to reference point22. Sensor elements 16 c and 16 d are connected in series, in a mannersimilar to sensor elements 16 a and 16 b, to respond to one component ofthe magnetic field. Sensor elements 18 c and 18 d are connectedsimilarly in series to respond to a second component of the magneticfield that is independent of the first component, and sensor elements 20c and 20 d are connected similarly in series to respond to a thirdcomponent of the magnetic field that is independent of the first twocomponents. Most preferably, sensor elements 16 c, 16 d, 18 c, 18 d, 20c and 20 d are sized and separated so that these three magnetic fieldcomponents are orthogonal. In practice, the cylindrical surfacewhereabout sensor elements 16 c, 16 d, 18 c, 18 d, 20 c and 20 d aredisposed could be the inner surface of probe 10 or the outer surface ofa cylindrical sleeve adapted to fit inside probe 10. In the case of thisembodiment of receiver 14 formed on the outer surface of a cylindricalsleeve, sensor elements 16 c, 16 d, 18 c, 18 d, 20 c and 20 d may befabricated by any one of several standard methods, includingphotolithography and laser trimming. FIG. 10 illustrates the preferredgeometry of sensor elements 16 c, 16 d, 18 c, 18 d, 20 c and 20 d: aflat rectangular spiral 17 of an electrical conductor 19. Only fourturns are shown in spiral 17, for illustrational simplicity. Preferably,however, there are several hundred turns in spiral 17. For example, aspiral 17, intended for a cylindrical surface of a diameter of 1.6millimeters, in which conductor 19 has a width of 0.25 microns, and inwhich the windings are separated by gaps of 0.25 microns, has 167 turns.

[0085]FIGS. 12A, 12B and 12C illustrate the distal end of a cardiaccatheter 300 of the present invention. FIG. 12A is a partly cut-awayperspective view of catheter 300 in the retracted position thereof. FIG.12B is a perspective view of catheter 300 in the extended positionthereof. FIG. 12C is an end-on view of catheter 300 in the retractedposition thereof. Catheter 300 includes a flexible cylindrical innersleeve 302 slidably mounted in a flexible cylindrical outer sleeve 304.Connecting distal end 306 of inner sleeve 302 to distal end 308 of outersleeve 304 are four flexible rectangular strips 310. When inner sleeve302 is in the extended position thereof relative to outer sleeve 304,strips 310 are flush against inner sleeve 302, as shown in FIG. 12B.When inner sleeve 302 is in the retracted position thereof relative toouter sleeve 304, strips 310 bow outward in circular arcs, as shown inFIG. 12A.

[0086] Catheter 300 includes a set of three orthogonal electromagneticfield component sensors 316, 318 and 320, in the manner of receiver 14of FIG. 1. First sensor 316 includes coils 316 a and 316 b mounted onopposite lateral edges 312 a and 314 a of strip 310 a and on oppositelateral edges 312 c and 314 c of strip 310 c. Coil 316 a is mounted onlateral edges 312 a and 312 c. Coil 316 b is mounted on lateral edges314 a and 314 b. Second sensor 318 includes coils 318 a and 318 bmounted on opposite lateral edges 312 b and 314 b of strip 310 b and onopposite lateral edges 312 d and 314 d of strip 310 d. Coil 318 a ismounted on lateral edges 312 b and 312 d. Coil 318 b is mounted onlateral edges 314 b and 314 d. Third sensor 320 includes coils 320 a and320 b. Inner sleeve 302 is cut away in FIG. 12A to show coils 320 a and320 b. For illustrational clarity, the wires of coils 316 a and 318 aare shown in FIGS. 12A and 12B as dashed lines, and only two turns areshown for each coil, although in practice at least nine turns of45-micron-diarneter copper wire are used. Note that the wires of coil316 a run through inner sleeve 302, from lateral edge 312 a to lateraledge 312 c, and do not terminate at the intersection of lateral edges312 a and 312 c with inner sleeve 302. Similarly, the wires of coil 318a do not terminate at the intersection of lateral edges 312 b and 312 dwith inner sleeve 302, but instead run from lateral edge 312 b tolateral edge 312 d. Also for illustrational clarity, lateral edges 312are shown much wider than they really are in preferred embodiments ofcatheter 300. Coils 320 a and 320 b are wound around a permeable core(not shown).

[0087] In a typical embodiment of catheter 300, the length of innersleeve 302 exceeds the length of outer sleeve 304 by 15.7 mm in theextended position. Also in a typical embodiment of catheter 300, each ofcoils 320 a and 320 b is about 1.1 mm long and about 1.1 mm in diameterand includes about 400 turns of 10 micron diameter copper wire.

[0088] Coils 320 a and 320 b are parallel and equidistant from a centralpoint 322. When catheter 300 is opened to the retracted positionthereof, as shown in FIGS. 12A and 12C, the circular arcs formed bystrips 310 are concentric with point 322. This makes coils 316 a, 316 b,318 a and 318 b circular and concentric with point 322, with coils 316 aand 316 b being mutually parallel, and with coils 318 a and 318 b beingmutually parallel, so that point 322 then becomes the reference pointfor electromagnetic field measurements.

[0089] In the extended position thereof, catheter 300 is thin enough,preferably less than about 2 mm in diameter, to be inserted via theblood vessels of a patient into the patient's heart. Once the distal endof catheter 300 is inside the desired chamber of the patient's heart,inner sleeve 302 is withdrawn relative to outer sleeve 304 to putcatheter 300 in the retracted position thereof. Sensors 316, 318 and 320are used in conjunction with transmitter 24 in the manner describedbelow to determine the location and orientation of the distal end ofcatheter 300 within the patient's heart.

[0090] Mounted on outward faces 324 of strips 310 are four electrodes326. Mounted on distal end 306 of inner sleeve 302 is an electrode 328.Electrodes 326 and 328 may be used for electrophysiologic mapping of thepatient's heart. Alternatively, high RF power levels may be applied toselected heart tissue via electrode 328 to ablate that tissue in thetreatment of conditions such as ventricular tachycardia.

[0091]FIGS. 13A and 13B illustrate the distal end of an alternativeembodiment 400 of the cardiac catheter of the present invention. FIG.13A is a partly cut-away side view of catheter 400 in the retractedposition thereof. FIG. 13B is an end-on view of catheter 400 in theretracted position thereof. Like catheter 300, catheter 400 includes aflexible cylindrical inner sleeve 402 slidably mounted in a flexiblecylindrical outer sleeve 404. Connecting distal end 406 of inner sleeve402 to distal end 408 of outer sleeve 404 is a single flexible member:an inflatable latex balloon 410. When inner sleeve 402 is in theextended position thereof relative to outer sleeve 404, balloon 410 isflush against inner sleeve 402. After the illustrated distal end ofcatheter 400 has been introduced to the targeted chamber of a patient'sheart, inner sleeve 402 is withdrawn to the retracted position thereof,and balloon 410 is inflated to assume a spherical shape.

[0092] Like catheter 300, catheter 400 includes a set of threeorthogonal electromagnetic field component sensors 416, 418 and 420, inthe manner of receiver 14 of FIG. 1. First sensor 416 includes parallelcoils 416 a and 416 b mounted as shown on outer surface 412 of balloon410. Second sensor 418 includes parallel coils 418 a and 418 b mountedorthogonally to coils 416 a and 416 b on outer surface 412, as shown.Third sensor 420 includes coils 420 a and 420 b. Balloon 410 and innersleeve 402 are cut away in FIG. 13A to show coils 420 a and 420 b. Coils420 a and 420 b are parallel and equidistant from a central point 422.When catheter 400 is opened to the retracted position thereof andballoon 410 is inflated to a spherical shape, outer surface 412 is asphere concentric with point 422. This makes coils 416 a, 416 b, 418 aand 418 b circular and concentric with point 422, so that point 422 thenbecomes the reference point for electromagnetic field measurements.

[0093] Also as in the case of catheter 300, catheter 400 includes fourelectrodes 426, similar to electrodes 326, mounted on outer surface 412,and an electrode 428, similar to electrode 328, mounted on distal end406 of inner sleeve 402.

[0094]FIG. 6 is a plan view of loop antennas 26, 28 and 30. Loop antenna26 is a rectangle with legs 26 a, 26 b, 26 c and 26 d. Loop antenna 28is a rectangle of the same shape and size as loop antenna 26, and withlegs 28 a, 28 b, 28 c and 28 d. Legs 26 b and 28 d are adjacent. Loopantenna 30 also is rectangular, with legs 30 a, 30 b, 30 c and 30 d. Leg30 a overlies legs 26 a and 28 a; leg 30 b overlies the upper half ofleg 28 b; and leg 30 d overlies the upper half of leg 26 d, so that loopantenna 30 overlaps half of loop antenna 26 and half of loop antenna 28.Also shown in phantom in FIG. 6 is a fourth rectangular loop antenna 46and a fifth rectangular loop antenna 48 that are not part of transmitter24 but are referred to in the explanation below. Loop antenna 46 is ofthe same shape and size as loop antenna 30, and overlaps the halves ofloop antennas 26 and 28 that are not overlapped by loop antenna 30. Loopantenna 48 matches the outer perimeter defined by loop antennas 26 and28.

[0095] To understand the preferred mode of the operation of the systemof the present invention, it is helpful to consider first a lesspreferred mode, based on time domain multiplexing, of operating asimilar system that includes all five loop antennas of FIG. 6. In thisless preferred mode, loop antenna 48 is energized using a sinusoidalcurrent of angular frequency col. Then, loop antennas 26 and 28 areenergized by oppositely directed sinusoidal currents of angularfrequency ω₁. Finally, loop antennas 30 and 46 are energized byoppositely directed sinusoidal currents of angular frequency ω₁. Theidea of this energization sequence is to produce, first, a field abovethe transmitter that is spatially symmetric in both the horizontal andthe vertical direction as seen in FIG. 6, then a field above thetransmitter that is antisymmetric in the horizontal direction andsymmetric in the vertical direction, and finally a field that issymmetric in the horizontal direction and antisymmetric in the verticaldirection. These three fields are linearly independent, and all threefields have significant amplitude all the way across the transmitter.The signals output by the three sensors of receiver 14 in response tothe electromagnetic waves so generated are sampled at times t_(m) byreception circuitry 34. The sampled signals are:

[0096] s⁰ _(lm)=c⁰ _(l,1) cos ω₁t_(m)+c⁰ _(i,2) sin ω₁t_(m) from loopantenna 48

[0097] s^(h) _(im)=c^(h) _(l,1) cos ω₁t_(m)+c^(h) _(l,2) sin ω₁t_(m)from loop antennas 26 and 28

[0098] s^(v) _(im)=c^(v) _(l,1) cos ω₁t_(m)+c^(v) _(l,2) sin ω₁t_(m)from loop antennas 30 and 46

[0099] where i indexes the sensor that receives the correspondingsignal. Coefficients c⁰ _(l,1), c^(h) _(l,1) and c^(v) _(l,1) are thein-phase amplitudes of the received signals. Coefficients c⁰ _(i,2),c^(h) _(l,2) and c^(v) _(l,2) are the quadrature amplitudes of thereceived signals. Because ω₁ is sufficiently low that receiver 14 is inthe near fields generated by the loop antennas, in principle thequadrature amplitudes should be identically zero. Because of inevitablephase distortions, for example in reception circuitry 34, the quadratureamplitudes generally are not zero.

[0100] Note that amplitudes c⁰ _(l,j), c^(h) _(l,j), and c^(v) _(l,j)(j=1,2)could be obtained by using only loop antennas 26, 28 and 30. Thesampled signals obtained by energizing loop antennas 26, 28 and 30separately with identical sinusoidal currents of angular frequency ω₁are:

[0101] s¹ _(lm)=c¹ _(l) cos ω₁t_(m)+c² _(l) sin ω₁t_(m) from loopantenna 26

[0102] s² _(lm) =c ³ _(i) cos ω₁t_(m)+c⁴ _(l) sin ω₁t_(m) from loopantenna 28

[0103] s³ _(lm)=c⁵ _(l) cos ω₁t_(m)+c⁶ _(l) sin ω₁t_(m) from loopantenna 30

[0104] the coefficients c¹ _(l), c³ _(l) and c⁵ _(l) being in-phaseamplitudes and the coefficients c² _(l), c⁴ _(l) and c⁶ _(l) beingquadrature amplitudes. Because the field radiated by loop antennas 26and 28 when identical currents J flow therein is the same as the fieldgenerated by loop antenna 48 when current J flows therein,

c ⁰ _(l,1) =c ¹ _(l) +c ³ _(l)  (1)

c ⁰ _(l,2) =c ² _(l) +c ⁴ _(l)  (2)

[0105] By definition,

c ^(h) _(i,1) =c ¹ _(l) −c ³ _(l)  (3)

c ^(h) _(l,2) =c ² _(i) −c ⁴ _(i)  (4)

[0106] Finally, the fact that the field radiated by loop antenna 48could also be emulated by identical currents flowing through loops 30and 46 gives

c ^(v) _(l,1)=2c ⁵ _(l) −c ¹ _(l) −c ³ _(l)  (5)

c ^(v) _(l,2)=2c ⁶ _(l) −c ² _(l) −c ⁴ _(l)  (6)

[0107] In the preferred mode of the operation of the system of thepresent invention, loop antennas 26, 28 and 30 are energizedsimultaneously with sinusoidal currents of angular frequencies ω₁, ω₂and ω₃, respectively. The sampled signals now are

s _(im) =c _(l1) cos ω₁ t _(m) +c _(l2) sin ω₁ t _(m) +c _(l3) cos ω₂ t_(m) +c _(l4) sin ω₂ t _(m) +c _(l5) cos ω ₃ t _(m) +c _(l6) sin ω₃ t_(m)  (7)

[0108] Note that now, amplitudes c_(l1) and c_(l2) refer to frequencyω₁, amplitudes c_(l3) and c_(l4) refer to frequency ω₂, and amplitudesc_(l5) and c_(l6) refer to frequency ω₃. The sampled signals areorganized in a matrix s of three rows, one row for each sensor ofreceiver 14, and as many columns as there are times t_(m) one column pertime. Amplitudes c_(ij) are organized in a matrix c of three rows andsix columns. The matrices s and c are related by a matrix A of six rowsand as many columns as there are in matrix s:

s=cA  (8)

[0109] Almost always, there are many more than six columns in matrix s,making equation (8) highly overdetermined. Because the transmissionfrequencies and the reception times are known, matrix A is known.Equation (8) is solved by right-multiplying both sides by a rightinverse of matrix A: a matrix, denoted as A⁻¹, such that AA⁻¹=I, where Iis the 6×6 identity matrix. Right inverse matrix A⁻¹ is not unique. Aparticular right inverse matrix A⁻¹ may be selected by criteria that arewell known in the art. For example, A⁻¹ may be the right inverse of A ofsmallest L² norm. Alternatively, matrix c is determined as thegeneralized inverse of equation (8):

c=sA ^(T)(AA ^(T))⁻¹  (9)

[0110] where the superscript “T” means “transpose”. The generalizedinverse has the advantage of being an implicit least squares solution ofequation (8).

[0111] In the special case of evenly sampled times t_(m), solvingequation (8) is mathematically equivalent to the cross-correlation of WO96/05768. Equation (8) allows the sampling of the signals from receiver14 at irregular times. Furthermore, there is no particular advantage tousing frequencies ω₁, ω₂ and ω₃ that are integral multiples of a basefrequency. Using closely spaced frequencies has the advantage ofallowing the use of narrow-band filters in reception circuitry 34, atthe expense of the duration of the measurement having to be at leastabout 2π/Δω, where Δω is the smallest frequency spacing, except in thespecial case of two signals of the same frequency and different phases.

[0112] Because receiver 14 is in the near field of transmitter 24,coefficients c_(lj) of equation (7) are the same as coefficients c^(l)_(i). It follows that equations (1)-(6) still hold, and either of two3×3 matrices M can be formed from the elements of matrix c for furtherprocessing according to the description in co-pending Israel PatentApplication 122578, an in-phase matrix $\begin{matrix}{M = \begin{bmatrix}c_{1,1}^{0} & c_{1,1}^{h} & c_{1,1}^{v} \\c_{2,1}^{0} & c_{2,1}^{h} & c_{2,1}^{v} \\c_{3,1}^{0} & c_{3,1}^{h} & c_{3,1}^{v}\end{bmatrix}} & (10)\end{matrix}$

[0113] or a quadrature matrix $\begin{matrix}{M = \begin{bmatrix}c_{1,2}^{0} & c_{1,2}^{h} & c_{1,2}^{v} \\c_{2,2}^{0} & c_{2,2}^{h} & c_{2,2}^{v} \\c_{3,2}^{0} & c_{3,2}^{h} & c_{3,2}^{v}\end{bmatrix}} & (11)\end{matrix}$

[0114] Note that because the system of the present invention is aclosed-loop system, there is no sign ambiguity in M, unlike thecorresponding matrix of co-pending Israel Patent Application 122578.

[0115] Let T be the orthonormal matrix that defines the rotation ofprobe 10 relative to the reference frame of transmitter 24. Write M inthe following form:

M=ET ₀ T  (12)

[0116] where T₀ is an orthogonal matrix and E is in general anonorthogonal matrix. In general, T₀ and E are functions of the positionof probe 10 relative to the reference frame of transmitter 24. Let

W ² =MM ^(T) =ET ₀ TT ^(T) T ₀ ^(T) E ^(T) =EE ^(T)  (13)

[0117] W² is real and symmetric, and so can be written asW²=Pd²P^(T)=(PdP^(T))², where d² is a diagonal matrix whose diagonalelements are the (real and positive) eigenvalues of W² and where P is amatrix whose columns are the corresponding eigenvectors of W². ThenW=PdP^(T)=E also is symmetric. Substituting in equation (12) gives:

M=PdP ^(T) T ₀ T  (14)

[0118] so that

T=T ₀ ^(T) Pd ⁻¹ P ^(T) M  (15)

[0119] If T₀ is known, then T, and hence the orientation of probe 10with respect to the reference frame of transmitter 24, can be computedusing equation (15).

[0120] For any particular configuration of the antennas of transmitter24, T₀ may be determined by either of two different calibrationprocedures.

[0121] In the experimental calibration procedure, probe 10 is orientedso that T is a unit matrix, probe 10 is moved to a succession ofpositions relative to transmitter 24, and M is measured at eachposition. The equation

T ₀ =Pd ⁻¹ P ^(T) M  (16)

[0122] gives T₀ at each of those calibration positions.

[0123] There are two variants of the theoretical calibration procedure,both of which exploit reciprocity to treat receiver 14 as a transmitterand transmitter 24 as a receiver. The first variant exploits theprinciple of reciprocity. The sensor elements are modeled as pointsources, including as many terms in their multipole expansions as arenecessary for accuracy, and their transmitted magnetic fields in theplane of transmitter 24 are calculated at a succession of positionsrelative thereto, also with probe 10 oriented so that T is a unitmatrix. The EMF induced in the antennas of transmitter 24 by thesetime-varying magnetic fields is calculated using Faraday's law. Thetransfer function of reception circuitry 34 then is used to compute M ateach calibration position, and equation (16) gives T₀ at eachcalibration position. In the second variant, the magnetic fieldgenerated by each antenna of transmitter 24 at the three frequencies ω₁,ω₂ and ω₃ is modeled using the Biot-Savart law. Note that each frequencycorresponds to a different sensor 16, 18 or 20. The signal received ateach sensor is proportional to the projection of the magnetic field onthe sensitivity direction of the sensor when object 10 is oriented sothat T is a unit matrix. This gives the corresponding column of M up toa multiplicative constant and up to a correction based on the transferfunction of reception circuitry 34.

[0124] To interpolate T₀ at other positions, a functional expression forT₀ is fitted to the measured values of T₀. Preferably, this functionalexpression is a polynomial. It has been found most preferable to expressthe Euler angles α, β and γ that define T₀ as the following 36-termpolynomials. The arguments of these polynomials are not direct functionsof Cartesian coordinates x, y and z, but are combinations of certainelements of matrix W that resemble x, y and z, specifically,a=W₁₃/(W₁₁+W₃₃), which resembles x; b=W₂₃/(W₂₂+W₃₃), which resembles y,and c=log(1/W₃₃), which resembles z. Using a direct product notation,the 36-term polynomials can be expressed as:

α=(a, a ³ , a ⁵)(b, b ³ , b ⁵)(1, c, c ² , c ³)Azcoe  (17)

β=(a, a ³ , a ⁵)(1, b ² , b ⁴ , b ⁶)(1, c, c ²)Elcoe  (18)

γ=(1, a ² , a ⁴ , a ⁶)(b, b ³ , b ⁵)(1, c, c ²)Rlcoe  (19)

[0125] where AZcoe, ELcoe and RLcoe are 36-component vectors of theazimuth coefficients, elevation coefficients and roll coefficients thatare fitted to the measured or calculated values of the Euler angles.Note that to fit these 36-component vectors, the calibration proceduremust be carried out at at least 36 calibration positions. At eachcalibration position, W is computed from M using equation (13), and theposition-like variables a, b and c are computed from W as above.

[0126] Similarly, the Cartesian coordinates x, y and z of probe 10relative to the reference frame of transmitter 24 may be expressed aspolynomials. It has been found most preferable to express x, y and z asthe following 36-term polynomials:

x=(a, a ³ , a ⁵)(1, b, b ⁴)(1, c, c ² , c ³)Xcoe  (20)

y=(1, a ² , a ⁴)(b, b ³ , b ⁵)(1, c, c ² , c ³)Ycoe  (21)

z=(1, a ² , a ⁴)(1, ² , b ⁴)(1, d, d ² , d ³)Zcoe  (22)

[0127] where Xcoe, Ycoe and Zcoe are 36-component vectors of thex-coefficients, the y-coefficients, and the z-coefficients,respectively; and d=log(c). As in the case of the Euler angles, theseposition coordinate coefficients are determined by either measuring orcomputing M at at least 36 calibration positions and fitting theresulting values of a, b and c to the known calibration values of x, yand z. Equations (17) through (22) may be used subsequently to infer theCartesian coordinates and Euler angles of moving and rotating probe 10noniteratively from measured values of M.

[0128] Although the antenna configuration illustrated in FIGS. 1 and 6is the most preferred configuration, other configurations fall withinthe scope of the present invention. FIGS. 7A, 7B and 7C show threealternative configurations of paired adjacent loop antennas 26′ and 28′.The arrows indicate the direction of current flow that emulates a singleloop antenna coincident with the outer perimeter of antennas 26′ and28′. Other useful coplanar overlapping antenna configurations aredescribed in PCT Publication No. WO 96/03188, entitled “Computerizedgame Board”, which is incorporated by reference for all purposes as iffully set forth herein.

[0129]FIG. 8 is a schematic block diagram of driving circuitry 32 fordriving a generic antenna 25 that represents any one of loop antennas26, 28 or 30. A digital signal generator 50 generates samples of asinusoid that are converted to an analog signal by a D/A converter 52.This analog signal is amplified by an amplifier 54 and sent to thepositive input 60 of a differential amplifier 58. Loop antenna 25 isconnected both to the output 64 of differential amplifier 58 and to thenegative input 62 of differential amplifier 58. Negative input 62 alsois grounded via a resistor 66. The feedback loop thus set up drivesantenna 25 at the frequency of the sinusoid generated by signalgenerator 50, and makes antenna 25 appear to be an open circuit at allother frequencies.

[0130] Unlike the circuitry of WO 97/36143, which acts to offset theinfluence of one loop antenna on another, the circuitry of FIG. 8decouples loop antenna 25 from the other loop antennas. The superiorityof the present invention over WO 97/36143 is evident. Consider, forexample, how WO 97/36143 and the present invention correct for themutual inductances of loop antenna 26, radiating at a frequency ω₁, andloop antenna 30, radiating at a frequency ω₂. The goal is to set up thefield of frequency ω₁ that would be present if only loop antenna 26, andnot loop antenna 30, were present, and to set up the field of frequencyω₂ that would be present if only loop antenna 30, and not loop antenna26, were present. By Faraday's and Ohm's laws, the time rate of changeof the magnetic flux through loop antenna 26 is proportional to thecurrent through loop antenna 26, and the time rate of change of themagnetic flux through loop antenna 30 is proportional to the currentthrough loop antenna 30. In the absence of loop antenna 30, loop antenna26 sets up a certain time-varying magnetic flux of frequency ω₁ acrossthe area that would be bounded by loop antenna 30 if loop antenna 30were present. The method of WO 97/36143 forces the time rate of changeof this magnetic flux through loop antenna 30 to be zero. Because themagnetic flux has no DC component, the magnetic flux itself through loopantenna 30 therefore also vanishes, which is contrary to the situationin the absence of loop antenna 30. By contrast, the present inventionmakes loop antenna 30 appear to be an open circuit at frequency ω₁ andso does not change the magnetic flux from what it would be in theabsence of loop antenna 30.

[0131]FIG. 9 shows, schematically, a C-mount fluoroscope 80 modifiedaccording to the present invention for simultaneous real-time imageacquisition and intrabody navigation. Fluoroscope 80 includes theconventional components of a C-mount fluoroscope: an x-ray source 82 andan image acquisition module 84 mounted on opposite ends of a C-mount 78,and a table 86 whereon the patient lies. Image acquisition module 84converting x-rays that transit the patient on table 86 into electronicsignals representative of a 2D image of the patient. C-mount 78 ispivotable about an axis 76 to allow the imaging of the patient fromseveral angles, thereby allowing the reconstruction of a 3D image of thepatient from successive 2D images. In addition, either a receiver 114,similar to receiver 14, or transmitter 24, is rigidly mounted on C-mount78. Receiver 114 or transmitter 24 serves to define a frame of referencethat is fixed relative to C-mount 78. The other components shown in FIG.1, i.e., driving circuitry 32, reception circuitry 34, andcontrol/processing unit 36, are connected to transmitter 24 and toreceiver 14 in probe 10 as described above in connection with FIG. 1. Inaddition, signals from receiver 114 that correspond to theelectromagnetic waves generated by transmitter 24′ are sent to receptioncircuitry 134 that is identical to reception circuitry 34, andcontroller/processor 36 directs the reception of received signals byreception circuitry 134 and the acquisition of an image of the patientby image acquisition module 84 of fluoroscope 80.

[0132] By determining the position and orientation of probe 10 relativeto the frame of reference defined by transmitter 24,controller/processor 36 determines the position and orientation of probe10 relative to each acquired 2D image. Alternatively, theelectromagnetic signals are transmitted by a transmitter 24′ that is notattached to C-mount 78, and controller/processor 36 determines theposition and orientation of probe 10 relative to the 2D images bydetermining the positions and orientations of receivers 14 and 114relative to transmitter 24′. Controller/processor 36 synthesizes acombined image that includes both the 3D image of the patient acquiredby fluoroscope 80 and an icon representing probe 10 positioned andoriented with respect to the 3D image of the patient in the same way asprobe 10 is positioned and oriented with respect to the interior of thepatient. Controller/processor 36 then displays this combined image on amonitor 92.

[0133] C-mount fluoroscope 80 is illustrative rather than limitative.The scope of the present invention includes all suitable devices foracquiring 2D or 3D images of the interior of a patient, in modalitiesincluding CT, MRI and ultrasound in addition to fluoroscopy.

[0134] Under certain circumstances, the image acquisition and theintrabody navigation may be done sequentially, rather thansimultaneously. This is advantageous if the medical imaging facilitiesand the medical treatment facilities can not be kept in the samelocation. For example, the human skull is sufficiently rigid that if areceiver of the present invention is rigidly mounted on the head of apatient using an appropriate headband, then the position and orientationof the receiver is a sufficient accurate representation of the positionand orientation of the patient's head to allow intracranial navigation.FIG. 11 shows a head 94 of a patient inside a (cut-away) CT scanner 98.As in the case of fluoroscope 80 of FIG. 9, receiver 114 and transmitter24 are rigidly attached to CT scanner 98, transmitter 24 being soattached via an arm 100. CT scanner 98 acquires 2D x-ray images ofsuccessive horizontal slices of head 94. A receiver 214 is rigidlymounted on head 94 using a headband 96. As the 2D images are acquired,the position and orientation of receiver 214 with respect to each imageis determined by the methods described above for determining theposition and orientation of probe 10 with respect to the 2D imagesacquired by fluoroscope 80. These positions and orientations are stored,along with the 2D images, in control/processing unit 36. Subsequently,during medical treatment of head 94 that requires navigation of probe 10through head 94, the position and orientation of probe 10 in head 94 isdetermined using signals from receivers 14 and 214 in the mannerdescribed above for positioning and orienting probe 10 with respect toC-mount 78 of fluoroscope 80 using receivers 14 and 114. Given, now, foreach 2D CT image, the position and orientation of probe 10 with respectto receiver 214 and the position and orientation of receiver 214 withrespect to that 2D image, it is trivial to determine the position andorientation of probe 10 with respect to that 2D image. As in the case ofthe simultaneous imaging and navigation depicted in FIG. 9,controller/processor 36 now synthesizes a combined image that includesboth the 3D image of head 94 acquired by CT scanner 98 and an iconrepresenting probe 10 positioned and oriented with respect to the 3Dimage of head 94 in the same way as probe 10 is positioned and orientedwith respect to head 94. Controller/processor 36 then displays thiscombined image on monitor 92.

[0135] As in the case of fluoroscope 80, CT scanner 98 is illustrativerather than limitative. The scope of the present invention includes allsuitable devices for acquiring 2D or 3D images of a limb of a patient,in modalities including MRI, ultrasound and fluoroscopy in addition toCT. Note that this method of image acquisition followed by intrabodynavigation allows the a centrally located imaging device to serveseveral medical treatment facilities.

[0136]FIG. 14 is a partially exploded, partial perspective view of aC-mount fluoroscope 80′ modified according to one aspect of the presentinvention. Like C-mount fluoroscope 80, C-mount fluoroscope 80′ includesan x-ray source 84 and an image acquisition module 82 at opposite endsof a C-mount 78. Image acquisition module 82 includes an imageintensifier 83, a front face 85 whereof faces x-ray source 84, and a CCDcamera 87, mounted on the end of image intensifier 83 that is oppositefront face 85, for acquiring images that are intensified by imageintensifier 83. Image intensifier 83 is housed in a cylindrical housing91. In addition, fluoroscope 80′ includes an annular compensator 500made of a magnetically permeable material such as mu-metal.

[0137] The need for compensator 500 derives from the fact that frontface 85 is electrically conductive. The electromagnetic waves generatedby transmitter 24 or 24′ induce eddy currents in front face 85 thatdistort the electromagnetic field sensed by receiver 14. Placing a massof a magnetically permeable substance such as mu-metal in the properspatial relationship with front face 85 suppresses this distortion. Thisis taught, for example, in U.S. Pat. No. 5,760,335, to Gilboa, whichpatent is incorporated by reference for all purposes as if fully setforth herein, in the context of shielding a CRT from external radiationwithout perturbing the electromagnetic field external to the CRT.

[0138] Preferably, compensator 500 is a ring, 5 cm in axial length, ofmu metal foil 0.5 mm thick. Compensator 500 is slidably mounted on theexternal surface 89 of cylindrical housing 91, as indicated bydouble-headed arrows 504, and is held in place by friction. It isstraightforward for one ordinarily skilled in the art to select aposition of compensator 500 on housing 91 that provides the optimalsuppression of distortions of the electromagnetic field outside imageintensifier 83 due to eddy currents in front face 85.

[0139] It often is desirable to retrofit a new apparatus such asreceiver 14 to an existing catheter rather than to design a new probe 10that includes both the new apparatus and the functionality of an alreadyexisting probe. This retrofit capability is particularly important ifprobe 10 would have been used for medical applications, and both theapparatus and the existing probe had already been approved for medicalapplications by the relevant regulatory bodies. Such a retrofitcapability then would preclude the need to obtain regulatory approvalfor the new probe, a process that often is both expensive andtime-consuming.

[0140]FIG. 16 illustrates just such a retrofit capability, for adaptinga satellite 550 to a substantially cylindrical catheter 552 forinvasively probing or treating a body cavity such as a chamber of theheart. Satellite 550 is an instrumentation capsule that may containreceiver 14 or any other medically useful apparatus. For example,satellite 550 may contain an apparatus for ablating cardiac tissue. Acatheter such as catheter 552 is introduced to the body cavity of apatient via the patient's blood vessels, via an introducer sheath. It isimportant that the external diameter of the introducer sheath beminimized, to reduce the risk of bleeding by the patient. Consequently,the external diameter of catheter 552 also must be minimized, and anyscheme for retrofitting satellite 550 to catheter 552 must allowsatellite 550 to be introduced into the introducer sheath along withcatheter 552. It is the latter requirement that generally precludessimply attaching satellite 550 to catheter 552. In addition, ifsatellite 550 includes receiver 14, with the intention of using receiver14 to track the position and orientation of catheter 550, then, whensatellite 550 and catheter 552 are deployed within the body cavity,satellite 550 must have a fixed position and orientation relative tocatheter 552.

[0141] The retrofitting scheme of FIG. 16 achieves these ends byproviding satellite 550 and catheter 552 with a mechanism for providingonly a loose mechanical connection between satellite 550 and catheter552 as satellite 550 and catheter 552 are introduced to the body cavity,and only then securing satellite 550 to catheter 552 at a fixed positionand orientation relative to catheter 552. FIG. 16A shows a thin flexibletether 554 attached to proximal end 556 of satellite 550. Tether 554provides a mechanical link to the outside of the patient. Depending onthe instrumentation installed in tether 554, tether 554 may also providea communications link to the outside of the patient. For example, ifsatellite 550 includes receiver 14, then extensions of wire pairs 38 areincluded in tether 554. Rigidly attached to tether 554 is a hollowcylindrical sleeve 558 whose inner diameter is the same as the outerdiameter of catheter 552.

[0142] The remainder of the mechanism for reversibly securing satellite550 to catheter 552 is shown in FIG. 16B. Catheter 552 is provided, neardistal end 564 thereof, with a pocket 560 made of a flexible, resilient,elastic material. Pocket 560 is attached rigidly to the outer surface ofcatheter 552. Pocket 560 includes an aperture 562, which is adjacentcatheter 552 at the proximal end of catheter 552, and which accommodatestether 554. Pocket 560 is sized to accommodate satellite 550 snuglytherein via an opening in distal end 566 of pocket 560.

[0143] Satellite 550, catheter 552 and the associated securing mechanismare assembled as shown in FIG. 16C, with tether 554 running throughaperture 562, sleeve 558 encircling catheter 552 proximal of pocket 560,and satellite 550 distal of pocket 560. Catheter 552 and tether 554 areshown emerging from the distal end of a protective jacket 568.Preferably, sleeve 558 is made of a low-friction material such asTeflon™, to allow sleeve 558 to slide freely along catheter 552. Theassembly shown in FIG. 16C is introduced to the introducer sheath withsatellite 550 in front of catheter 552. During this introduction, pocket560 is compressed against the outer surface of catheter 552 by theintroducer sheath. Tether 554 is sufficiently flexible to bend alongwith catheter 552 and jacket 568 as the assembly shown in FIG. 16Cpasses through the patient's blood vessels, but is sufficiently rigid topush satellite 550 ahead of distal end 564 of catheter 552 as catheter552 is inserted into the patient. As a result, satellite 550 and distalend 564 of catheter 552 reach interior of the targeted body cavity inthe configuration illustrated in FIG. 16C. At this point, pocket 560opens, and tether 554 is pulled to withdraw satellite 550 into pocket560 via the opening in distal end 566 of pocket 560. Satellite 550 andtether 554 now are held by pocket 560, sleeve 558 and jacket 568 in afixed position and orientation relative to catheter 552, as illustratedin FIG. 16D.

[0144] Subsequent to treatment, tether 554 is pushed to restore theconfiguration shown in FIG. 16C, to allow catheter 552 and satellite 550to be withdrawn from the patient.

[0145] While the invention has been described with respect to a limitednumber of embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

What is claimed is:
 1. A system for navigating a probe inside a body,comprising: (a) a receiver of electromagnetic radiation, inside theprobe; (b) a device for acquiring an image of the body; and (c) atransmitter, of said electromagnetic radiation, including at least oneantenna rigidly attached to said device so as to define a frame ofreference that is fixed with respect to said device.
 2. The system ofclaim 1, wherein said system further comprises: (d) a mechanism fordisplaying a representation of the probe superposed on at least aportion of said image.
 3. The system of claim 1, wherein said image isselected from the group of modalities consisting of CT, MRI, ultrasoundand fluoroscopy.
 4. A system for navigating a probe inside a body,comprising: (a) a first receiver of electromagnetic radiation, insidethe probe; (b) a device for acquiring an image of the body; and (c) asecond receiver, of said electromagnetic radiation, rigidly attached tosaid device so as to define a frame of reference that is fixed withrespect to said device.
 5. The system of claim 4, further comprising:(d) a transmitter of said electromagnetic radiation.
 6. The system ofclaim 4, further comprising: (d) a mechanism for displaying arepresentation of the probe superposed on at least a portion of saidimage.
 7. The system of claim 4, wherein said image is selected from thegroup of modalities consisting of CT, MRI, ultrasound and fluoroscopy.8. A method of navigating a probe inside a body, comprising the stepsof: (a) providing a device for acquiring an image of the body; (b)simultaneously: (i) acquiring said image of the body, and (ii)determining a position and orientation of the probe with respect to saidimage; and (c) displaying said image of the body with a representationof the probe superposed thereon according to said position and saidorientation.
 9. The method of claim 8, wherein said image is selectedfrom the group of modalities consisting of CT, MRI, ultrasound andfluoroscopy.
 10. The method of claim 8, wherein said determining of saidposition and said orientation is effected by steps including: (A)transmitting electromagnetic radiation, using a transmitter; (B)receiving said electromagnetic radiation, using a first receiver insidethe probe, thereby producing signals corresponding to saidelectromagnetic radiation; and (C) inferring said position and saidorientation from said signals.
 11. The method of claim 10, wherein saidtransmitter includes at least one antenna rigidly attached to saiddevice so as to define a frame of reference that is fixed with respectto said device.
 12. The method of claim 10, wherein said determining ofsaid position and said orientation is effected by steps including: (D)receiving said electromagnetic radiation, using a second receiverrigidly attached to said device so as to define a frame of referencethat is fixed with respect to said device.
 13. A method of navigating aprobe inside a body, comprising the steps of: (a) providing a device foracquiring an image of the body; (b) simultaneously: (i) acquiring saidimage of the body, and (ii) determining a position and an orientation ofthe body with respect to said image; (c) determining a position and anorientation of the probe with respect to the body; and (d) displayingsaid image of the body with a representation of the probe superposedthereon according to both of said positions and both of saidorientations.
 14. The method of claim 13, wherein said image is selectedfrom the group of modalities consisting of CT, MRI, ultrasound andfluoroscopy.
 15. The method of claim 13, wherein said determining ofsaid position and said orientation of the body with respect to saidimage is effected by steps including: (A) transmitting electromagneticradiation, using a transmitter; (B) receiving said electromagneticradiation, using a first receiver rigidly attached to the body, therebyproducing signals corresponding to said electromagnetic radiation; and(C) inferring said position and said orientation of the body withrespect to said image from said signals.
 16. The method of claim 15,wherein said transmitter includes at least one antenna rigidly attachedto said device so as to define a frame of reference that is fixed withrespect to said device.
 17. The method of claim 15, wherein saiddetermining of said position and said orientation of the body withrespect to said image is effected by steps including: (D) receiving saidelectromagnetic radiation, using a second receiver rigidly attached tosaid device so as to define a frame of reference that is fixed withrespect to said device.
 18. The method of claim 13, wherein saiddetermining of said position and said orientation of the probe withrespect to the body is effected by steps including: (i) transmittingelectromagnetic radiation, using a transmitter; (ii) receiving saidelectromagnetic radiation, using a first receiver rigidly attached tothe body, thereby producing signals representative of a position and anorientation of the body with respect to said transmitter; (iii)receiving said electromagnetic radiation, using a second receiver insidethe probe, thereby producing signals representative of a position and anorientation of the probe with respect to said transmitter; and (iv)inferring said position and said orientation of the probe with respectto the body from said position and said orientation of the body withrespect to said transmitter and from said position and said orientationof the probe with respect to said transmitter.